Exemplary embodiments relate generally to the internal voltage generator of a semiconductor memory device and, more particularly, to the internal voltage generator of a semiconductor memory device, having a potential changed according to a temperature change.
Recently, the demand for a non-volatile memory device, which retains data stored in the device even in absence of power supply, is increasing.
The nonvolatile memory device can be electrically programmed and erased. According to a known art, a floating gate is interposed between a control gate and a channel of a transistor of a nonvolatile memory cell. Also, according to a known programming mechanism, electrons tunnel through a barrier, e.g., an insulating layer of the floating gate in the presence of a high electric field, and the electrons placed on the floating gate are trapped therein. The electrons of the floating gate would not discharge for long periods of time and shift the threshold voltage of the transistor.
During a program verification operation or a read operation for the nonvolatile memory cells, whether the threshold voltage Vth of the memory cell is above a certain verification voltage or a certain read voltage is determined.
However, the threshold voltage Vth of the memory cell may vary according to temperature, and if the program verification operation or the read operation is performed with a fixed verification voltage or a fixed read voltage, data stored in the memory cell may be read incorrectly. Further, in the case of a multi-level cell (MLC) program method, because distances of each threshold voltage distributions are closer than that of a single-level cell (MLC) program method.
Therefore, the program verification operation or the read operation may be performed with a verification voltage or a read voltage variable according to temperature.
FIG. 1 is a graph illustrating current and voltage characteristics according to temperature, and FIG. 2 shows changes of threshold voltage distributions of memory cells.
Referring to FIGS. 1 and 2, electric characteristics of a semiconductor device, such as an MOS and a BJT, may vary according to temperature. According to a known art, as temperature is lowered within a temperature range above a point where a cold temperature curve and a hot temperature curve meet, the threshold voltage of the MOS transistor increases and the amount of current decreases. However, as temperature is lowered within a temperature range below the point where the cold temperature curve and the hot temperature curve meet, the threshold voltage of the MOS transistor decreases and the amount of current increases. Here, the amount of current at the cold temperature is equal to the amount of current at the hot temperature at the point where the cold temperature curve and the hot temperature curve meet, and the current at the point may be called a zero point current. According to a known art, a sense current used in a nonvolatile memory device is about 100 nA to 200 nA, and the sense current is lower than the zero point current. Accordingly, the threshold voltage distributions of memory cells decrease as temperature increases. Further, in the read operation of the nonvolatile memory device, the threshold voltages of the memory cells may vary according to temperature. Consequently, a probability of occurrence of an error may increase.